Because a paper's first paragraph is often difficult to write,
many students have learned a 'first paragraph formula'. According to this
formula, you start the first paragraph with a thesis statement or question,
add two to four supporting statements, and end the first paragraph with a
concluding statement. Because all or nearly all of these first paragraph sentences
represent topic sentences from the paper's other paragraphs, the first paragraph
acts as a summary or even an abstract.

This kind of summary first paragraph may be useful because
it does provide some structure, and some structure is better than no structure.
But the summary first paragraph is usually ineffective because it introduces
ideas without providing necessary context or background - the reader sees
the ideas but doesn't really understand them. Moreover, when the same phrases/ideas
appear in paragraph 1 and again in paragraphs 2 - 5, the paper sounds redundant.
The sense of redundancy is especially strong with short essays (4 pages or
less).

If you are writing a short essay, do not use the first paragraph
to summarize. Instead, use the first paragraph to describe the problem or
question. And then develop and analyze that problem and question in the following
paragraphs. Finish with some conclusion but avoid repeating the points you
just made - in a short paper, this repetition will sound redundant and will
therefore be ineffective.

If you are writing a longer paper, do not try to introduce
everything in the first paragraph. Instead, use three or more paragraphs as
an introduction. Each paragraph should provide enough context or background
so that that the reader fully understands where the paper is going while he
or she is reading that paragraph; in other words, the reader should not have
to read paragraph 4 to reasonably understand sentences in paragraph 1.

Of course, each of these paragraphs may require additional
paragraphs, so don't infer that there is some proper number of paragraphs.

Having codified this pattern by writing it down, I now worry
that it will produce formulaic and therefore dull introductions. Although
that is possible, the diversity and complexity of subject matter should usually
color or mask the underlying structure so that the reader pays more attention
to the subject matter than to paragraph arrangement. On the other hand, other
patterns for introductions could be more effective, and effective writers
will enjoy finding them.